chore(deps): defer Prisma major updates pending coordinated upgrade ADR

Renovate kept proposing Prisma 7 even though we deliberately
downgraded to Prisma 6 in PR #3 — `nestjs-prisma@0.27.0` is
incompatible with Prisma 7's driver-adapter contract, and the
upgrade is non-trivial enough (PrismaClient instantiation, request-
scoped lifecycle, schema validation) to warrant its own ADR rather
than a silent Renovate merge.

Add an `enabled: false` packageRule for `matchUpdateTypes: ["major"]`
on `prisma`, `@prisma/*`, `nestjs-prisma`. Patch and minor bumps of
the 6.x line keep flowing.

Document the pin and its triggers in ADR-0006:
- New "Prisma version pin: 6.x in v1" subsection — explains the
  current narrowing of "latest stable major" to 6.x and the two
  triggers for revisiting (nestjs-prisma catches up, or we drop the
  wrapper).
- §Confirmation updated to surface the pin and the Renovate rule.

The matching open PR proposing Prisma 7 will be closed manually on
Gitea; once this lands, Renovate stops re-creating it.

renovate.json

@@ -38,6 +38,12 @@
       "groupName": "Prisma",
       "matchPackageNames": ["prisma", "@prisma/*", "nestjs-prisma"]
     },
+    {
+      "matchPackageNames": ["prisma", "@prisma/*", "nestjs-prisma"],
+      "matchUpdateTypes": ["major"],
+      "enabled": false,
+      "description": "Prisma 7 requires a coordinated upgrade with nestjs-prisma compatibility (Prisma 7 was downgraded to 6 in PR #3 because nestjs-prisma 0.27.0 is incompatible with Prisma 7). Re-enable once a Prisma 7 ADR lands and the wrapper situation is settled — see ADR-0006 §'Prisma version pin: 6.x in v1'."
+    },
     {
       "groupName": "Vitest",
       "matchPackageNames": ["vitest", "@vitest/*", "/^vitest-/"]